You can pay for cycles using the icp cli tool mentioned above. The documentation is here: Tokens and Cycles | ICP CLI
You would type something like
icp canister top-up my-canister --amount 1T -e ic
You can buy the cycles from an exchange like icpswap and them send them to the PID for your icp-cli identity, or you can mint them with ICP using the cli tool as well.
Additional options are out there such as automatic top-up services. I recently released one (you can read more about it here: Beta release of Unicycle top up service ) or just check it out directly at https://ic-unicycle.com/. This tool looks at the minting price and swap prices and automatically chooses the best deal for you. You can configure options for how low you want to allow your canisters to get before topping them up and how many cycles you want to top up with each time it dips below that value. There is a 5% fee for top ups, but it earns you credits that will reduce or fully cancel out the fee over time (again - full details in the linked post above).